The present invention relates to a process for the manufacture of a lithographic printing form by electrophotographic means.
Processes of this kind are known and are, e.g., described in German Pat. No. 11 17 391, German Offenlegungsschrift No. 23 22 047, and German Offenlegungsschrift No. 25 26 720. In conventional embodiments of the process, a photoconductive layer which has been applied to an electrically conductive layer support is charged, exposed image-wise, and developed with liquid or dry toner to give an image. The toner image is then fixed by heating, and finally the printing plate is decoated by removing the unprotected background areas of the photoconductive layer. The offset printing form thus obtained is ink-receptive in the toner image areas and water-receptive in the bared areas of the support surface.
It has shown that, especially in larger continuous full shade areas, printing forms prepared in this way have a tendency to exhibit voids where no ink is transferred. When examining the causes, it was found that the toner, which usually contains a resin which is soluble in organic solvents, most probably swells and gets sticky when it remains in contact with ink for a longer period of time. Then, the toner is able to hold dirt particles or to form an uneven, wrinkled surface.